With the development of radio and space technologies, several satellites based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European SPS such as the proposed “Galileo” program. The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time. In this application, we use the term Navigation Satellite System (NSS) to encompass any type of satellite-based communication system used for navigation, specifically terrestrial navigation, by a GPS receiver. The GPS receiver is typically included in a navigation device, that may be personal navigation device (PND).
Improving the positional accuracy calculated by a navigation device becomes more of a necessity in environments where satellite signals are degraded, and, as a result, the GPS receiver frequently encounters problems in locking onto the signals that are needed for the calculation of position, velocity, altitude, and time. In a degraded signal environment (e.g., a signal environment where signal strength is below 28 dBHz), satellite signals can be weak or otherwise difficult for GPS receivers to lock on to. Degraded signal environments are often encountered in urban areas, such as cities with many tall buildings. A city with many tall buildings contains “urban canyons”, which are environments where streets cut through dense blocks of structures such as skyscrapers. In urban canyons, satellite signals are frequently not visible or are degraded due to the signals being partially or fully blocked by buildings, for example. Consequently, the problem of inaccurate position calculations by GPS receivers in degraded signal environments is especially acute in urban areas, which not only has tall structures, but also has underground infrastructure, such as subway trains, tunnels, underpasses, underground parking lots, basements etc. Known measurement errors in degraded signal environment includes multi-path errors, cross-correlation errors, etc., which in turn translate to navigation errors.
In some conventional systems and methods, dead-reckoning (DR) sensors are integrated with a GPS receiver to augment the satellite-signal based position calculation. One such example is described in co-owned U.S. Pat. No. 7,756,639, entitled, “System and Method for Augmenting a Satellite-Based Navigation Solution,” to Colley et al. However, integrated GPS/DR systems also have limitations due to long-term growth of DR errors and dependence of DR sensors on external conditions.
One way to improve the accuracy of a calculated GPS position (with or without DR sensor) is to make accuracy improvements with the aid of a map database. Some attempts have been made to provide cartography information from this map database back to the GPS receiver in real-time to aid in the receiver's navigation solution. However, many of the conventional approaches use a full-fledged map-matching procedure, which is usually performed outside the navigation receiver. External map matching with a feedback to the navigation chip is possible, but may not be the optimal solution due to interface and cost issues. There is a possibility that only limited information can be exchanged across the interface. Also, in most cases, navigation vendors have worked independent of vendors who provide mapping databases, leaving room for further integration.
Co-pending co-owned U.S. patent application Ser. No. 12/409,315, filed Mar. 23, 2009, titled, “Method and Apparatus for Improving GPS Positioning Using an Embedded Map Database,” which is published as US 2011/0241935, provides one approach towards integration, where map information is embedded within a GPS receiver. However, embedding a map database in the GPS receiver itself may lead to bulkier navigation device size. In order to optimize the size of the navigation device, size of the map database itself and/or complexity of the map-matching or navigational algorithms need to be optimized.
Accordingly, techniques and devices for making better accuracy improvements to a navigation receiver's position calculations in degraded signal environments remain desirable, where selective information can be extracted from a map database that is optimized for providing positional correction in unison with a navigational routine executed by the navigation receiver.